The present invention relates to improvements in a method of fabricating a superconducting cavity and, more particularly, to an inexpensive fabrication of superconducting cavities characterized by a thin superconductive film having excellent resonance characteristic, by using a PVD (Physical Vapor Deposition) method.
Superconducting cavities (cavity resonator) have been heretofore fabricated by the following steps:
(1) Three types of elements 2, 2', and 3 for cavities, are formed by spinning, drawing, or grinding a niobium (Nb) plate, as shown in FIG. 1. The elements are welded by an electron beam welding method, the inner surfaces of the elements are then electrolytically polished with a mixture of fluoric acid and sulfuric acid, to smoothen the inner surfaces, and impurities are removed, to provide superconducting cavities 1.
In order to improve the smoothness of the inner surfaces of the elements as much as possible, the elements are welded by emitting an electron beam from the inside. Since niobium has a high melting point of 2415.degree. C. and excellent corrosion resistance, it is difficult to form a thin film by an electric plating, in order to provide a plated film of high quality.
(2) A niobium pipe is expanded in diameter by a hydraulic bulging method, to provide a product. Since the niobium pipe cannot be expanded to the necessary shape, in one step, the pipe must be slightly expanded while annealing in a vacuum, during the course of the step.
Since the niobium layer is thus formed by welding, a stepped portion can be feasibly formed on the welded portion, thereby deteriorating the high-frequency, and the superconducting characteristics. Since niobium having low thermal conductivity is used and the niobium layer may be as thick as several mm, if the film has a defect or contains any foreign matter, high-frequency loss heat can be dissipated only with difficulty, thereby causing the superconducting characteristics to decrease. Since the niobium is heated to high temperatures by annealing and welding, it therefore tends to be oxidized. In order to prevent the niobium from oxidizing, vacuum annealing and electron beam welding are required, thereby resulting in a high cost. Since there is a danger of damaging the superconducting state due to the introduction of impurities such as iron or dusts from the surface of the mechanical forming niobium, it is necessary to electrolytically polish it with a mixture of fluoric acid and sulfuric acid, electrolytic polishing is dangerous and involves complicated steps, thereby causing reduced productivity.
The inventors of the present invention have previously reduced the above-mentioned drawbacks, and proposed a method of fabricating a superconducting cavity comprising the steps of forming a hollow core made of aluminum or aluminum alloy having a cross-shaped section, shaping the size of the outside of the core to be the same as the inside of a desired superconducting cavity, contacting a thin niobium film on the outer periphery of the core, then forming a copper coating layer through a thin copper film on the outer periphery, in the shape of the cavity, and melting the core of aluminum or aluminum alloy (Japanese Patent Application No. 116691/1984), and a method of fabricating a superconducting cavity comprising the steps of forming a core of an aluminum or aluminum alloy pipe having a desired shape, forming a thin niobium film on the outer periphery, then forming a copper coating layer through a thin copper film on the outside, expanding the core in diameter and them melting and removing the core (Japanese Patent Application No. 116692/1984).
The reason aluminum or aluminum alloy (hereinafter referred to "aluminum") is used in this method is because it has suitable strength in the case of fabricating the core, it has high workability, thus improving the stability of the shape, and has preferable smoothness as well as having a high melting velocity in the melting and removing steps of the aluminum core in the later steps. These qualities thus make aluminum economical to use in this process. However, since the thin niobium film is, on the contrary, formed directly on the outer periphery of the aluminum, the niobium is embrittled on the outer periphery by hydrogen generated at the time of reacting with the aluminum, in the step of melting the aluminum with hydrochloric acid, as described above, and even if the aluminum is melted and removed, the superconducting characteristic of the niobium is remarkably deteriorated.